1. Field of the Invention
The present invention relates to a soft vinyl chloride resin composition having a wide usable temperature range and excellent heat resistance, low temperature resistance, and long-term durability. The present invention also relates to automobile components such as covers of air bag devices, steering wheel pads, shift lever boots and assist grips, hoses, films, artificial leathers, gaskets and conductor coverings made of this soft vinyl chloride resin composition.
2. Description of the Related Art
Soft vinyl chloride resin is tough and has excellent flame retardancy. Therefore, it has been widely used for automobile components such as covers of air bag devices, steering wheel pads, shift lever boots and assist grips, hoses, films, artificial leathers, gaskets and conductor coverings.
It is required that the soft vinyl chloride resin used for the steering wheel pad have excellent heat resistance, migration resistance and low volatility.
It is also required that the steering wheel pad have a sufficient flexibility without becoming brittle even at low temperatures, for example, about -40.degree. C., as well as the above listed properties of the soft vinyl chloride resin. The energy absorbing mechanisms and the air bag mechanisms recently adopted to a steering wheel pad have an added requirement of enhancing safety.
An approach to improve the properties of the vinyl chloride resin at low temperatures includes adding a plasticizer. It is known that the glass transition temperature (Tg) can be freely controlled depending upon the kind of the plasticizer and the amount of plasticizer. However, to provide enough plasticizer to provide a low temperature flexibility, even at -40.degree. C., will cause problems in the surface stickiness and the heat resistance of the resin, and bleeding and volatility of the plasticizer.
To overcome these problems, it is known that some or all of the plasticizer can be replaced with a thermoplastic elastomer having an excellent low temperature resistance.
Particularly, adding a thermoplastic polyurethane resin has been proposed by, for example, Japanese Unexamined Patent Publications Nos. Sho 55-116747, Sho 52-108454, Sho 53-18656, Sho 60-92345, since the resin has a good compatibility with the vinyl chloride resin.
However, these approaches do not satisfy the requirements for low temperature resistance, the balance between low temperature resistance and heat resistance, the volatility resistance and the bleed resistance. No composition which has a wide usable temperature range and is excellent in both heat resistance and low temperature resistance has yet been discovered.
In view of these points, the present applicant has proposed a soft vinyl chloride resin in which a thermoplastic polyurethane resin and a sebacic acid plasticizer are mixed with a vinyl chloride resin at a given ratio, Japanese Patent Application No. Hei 1-279455.
The soft vinyl chloride resin has the following problems. The soft vinyl chloride resin has a peak temperature of tan .delta. (loss factor) of not higher than -15.degree. C., which is a measure of the glass transition temperature (Tg). The vinyl chloride resin exhibits no brittle loss of the material, even at -40.degree. C. and has a tensional modulus at higher temperatures. Accordingly, it is excellent in physical properties such as flexibility and modulus in a wide range from a low temperature of -40.degree. C. to a high temperature of 100.degree. C. in comparison with conventional soft vinyl chloride resins.
However, since this resin has a high volatility weight loss on thermal aging, it cannot be applied for functional components such as steering wheel pads requiring high demands for durability and heat resistance.